Commerical, or civil, aircraft are conventionally designed for reducing exhaust emissions from combustion of hydrocarbon fuels such as, for example, Jet A fuel. The exhaust emissions may include hydrocarbon particulate matter, in the form of smoke, for example, carbon monoxide, and nitrogen oxides (NO.sub.x) such as, for example, nitrogen dioxide NO.sub.2. NO.sub.x emissions are known to occur from combustion at relatively high temperatures, for example over 3000.degree. F. (1648.degree. C.). These temperatures occur when fuel is burned at fuel-air ratios at or near stoichiometric. The amount of emissions formed is directly related to the time that combustion takes place at these conditions.
Conventional gas turbine engine combustors for use in an engine for powering an aircraft are conventionally sized and configured for obtaining varying fuel/air ratios during the varying power output requirements of the engine such as, for example, during light-off, idle, takeoff, and cruise modes of operation of the engine in the aircraft. At relatively low power modes, such as at light-off and idle, a relatively rich fuel/air ratio is desired for initiating combustion and maintaining stability of the combustion. At relatively high power modes, such as for example cruise operation of the engine in the aircraft, a relatively lean fuel/air ratio is desired for obtaining reduced exhaust emissions.
In the cruise mode, for example, where an aircraft gas turbine operates for a substantial amount of time, conventional combustors are typically sized for obtaining combustion at generally stoichiometric fuel/air ratios in the dome region, which represents theoretically complete combustion. However, in practical applications, exhaust emissions nevertheless occur, and conventional combustors utilize various means for reducing exhaust emissions.
Furthermore, aircraft intended to be operated at relatively high speed and at high altitude require engines having higher performance and power output. This may be accomplished by increasing the operating temperature of the engine cycle. These higher cycle temperatures will result in higher combustion zone temperatures and a higher NO.sub.x emissions formation rate. Therefore, in a conventional engine, NO.sub.x levels will increase which is especially undesirable at high altitudes for its potential damage to the ozone layer.